Field of the Invention
The present invention is related to an arrangement for a passive infrared motion detector in which a plurality of reflecting surfaces are arranged to direct beams of infrared radiation coming from a plurality of separate directions and focus these beams on a detector.
An optical arrangement for reflecting and focusing separate beams of infrared radiation, which are coming from a plurality of separate directions to be monitored, onto an infrared detector is known and examples are described in German AS No. 21 03 909, which corresponds to British Pat. No. 1,335,410. The infrared motion detector will register the intrusion of a person into a space, that is being monitored, or an object moving in the space by detecting the disturbance of the infrared radiation of the background which is caused by the moving object or intrusion. An electronic device evaluates the characteristic change of the infrared radiation applied to the detector and this evaluation can be used to actuate an alarm.
For a signal disturbance interval, which is as large as possible, the space is divided into a plurality of fields of view, lens coverage areas, or monitoring areas so that the monitoring areas are separated by dark field areas with both areas being solid angles with the monitoring areas being taken in by the detector and the dark field areas not being monitored. The motion of an object to be registered from a monitored area into a dark field area and vice versa will cause an evaluatable change of the infrared radiation striking the detector.
According to the above mentioned British patent (German patent), the division of the space to be monitored into a plurality of monitoring means, fields of view or lens coverage is achieved with the assistance of either a compound optic or using a cone shell-shaped mirror aperture, which marks out a compass or circular band of space to be monitored. Small concave mirrors or respectively sections of concave mirrors, which have a relatively short focal length, are arranged to focus on a common point at which the detector unit is positioned. Each of these concave mirrors covers radiation from a given direction to be monitored separate or lens coverage area or field of view. Such a compound optic has a few disadvantages which are as follows:
1. The greatest angle, which can be monitored with a compound optic, is approximately 90.degree. because the angle of incidence of radiation on the detector unit is approximately the same as the angle of incidence into the device and because the standard detector unit only supplies small signals when receiving radiation from a large angle of incidence. However, it is often desirable for many uses to be able to monitor a significantly larger angular range than 90.degree..
2. Because of the short focal length, the electrical signal generated by the detector greatly depends on the distance of the body that is being detected from the detector unit. When the amplification is adjusted in such a manner that sufficient signals are still received for great distances, the close range is strongly overweighted so that thermal disturbances immediately in front of the device, which disturbances may be caused by insects flying by the device or rising warm air, could lead to a false alarm under unfavorable conditions. In order to increase the focal length of the compound optics, the housing dimensions for the device would have to be greatly enlarged; and
3. Another disadvantage of the compound optics is the crossing over of the radiation bundles or beams from various lens coverage areas or fields of view. The locally excessive sensitivity at such cross over points further increases the danger of a false alarm as discussed hereinabove. Although the cross over points could be positioned behind the front plate and within the interior of the housing of the device, the radiation bundles would still lie close to one another immediately in front of the front plate of the housing so that a thermal disturbance could easily fall into a number of lens coverage areas at the same time and, therefore, could trigger a false alarm.